In product assembly lines having cardboard boxes filled with a product, the boxes, or cases, or workpieces, are closed and glued using an automated system. The cases may be filled with a granular product such as cereal, for example. The cases move along a filling line and adhesive is applied to the case flaps. The flaps are closed, and the adhesive sets quickly. A typical apparatus and method currently used to apply the adhesive to the cases is shown in FIGS. 1-3. The adhesive is typically a hot-melt type of glue.
FIG. 1 depicts an exemplary production dispensing system at 10. The adhesive is stored in a reservoir 12, and then fed into a pump 14. The pump 14 delivers the adhesive to an applicator nozzle and valve 38. The applicator 38 includes an electrically controlled actuator 36, which facilitates dispensing timing and duration. The actuator 36 is in turn controlled by a dispenser control computer 42. Production sensors 44 on the filling line (upstream or downstream) detect workpieces 40 moving through in a process direction 46, and the signals are sent to the control 42. In FIG. 1, dotted lines represent electrical control signals. Solid lines, represent fluid conduits and connections. The reservoir 12, pump 14, nozzle 38, and associated piping are maintained at a predetermined temperature to prevent the hot-melt glue from solidifying in the system. The production system shown in FIG. 1 could make use of the remote adhesive monitoring system.
The pump 14 is typically a double-acting piston type. Each end of the cylinder, first chamber 18A and second chamber 18B, has two check valves. First chamber 18A has one first inlet check valve 24 from the reservoir 12, and one first outlet check valve 26 connected to the nozzle 38. Second chamber 18B has one second inlet check valve 28 from the reservoir 12, and one second outlet check valve 30 connected to the nozzle 38. From the pump 14, the adhesive flows to nozzle 38, and is then extruded onto the workpieces 40. Typically, the adhesive is extruded in a series of beads of a predetermined length and cross-section, resulting in a uniform weight of each bead for a given production run.
FIG. 2 shows the pump 14 with the piston 16 moving to the left as in arrow 20. First chamber 18A has positive gauge pressure (above atmospheric). The pressure will close first inlet check valve 24. First outlet check valve 26 will open, and adhesive will flow to the nozzle 38. Second chamber 18B has negative gauge pressure (below atmospheric). The pressure will close second outlet check valve 30. Second inlet check valve 28 will open, and adhesive will flow from the reservoir 12 into the second chamber 18B. Due to manufacturing tolerances, there is a significant gap between the piston 16 and the cylinder bore 18C. The pressure difference will cause adhesive to flow, or leak, around the piston 16 from first chamber 18A to second chamber 18B. The leakage is shown at 34.
FIG. 3 shows the pump 14 with the piston 16 moving to the right as in arrow 22. Second chamber 18B has positive gauge pressure. The pressure will close second inlet check valve 28. Second outlet check valve 30 will open, and adhesive will flow to the nozzle 38. First chamber 18A has negative gauge pressure. The pressure will close first outlet check valve 26. First inlet check valve 24 will open, and adhesive will flow from the reservoir 12 into the first chamber 18A. Due to the gap between the piston 16 and the cylinder bore 18C, the pressure difference will cause adhesive to flow, or leak, around the piston 16 from second chamber 18B to first chamber 18A. The leakage is shown at 32.
A movement of the piston 16 in one direction from bottom dead center at one end of the cylinder to top dead center at the opposite end is called one stroke. A movement of the piston 16 from any starting position to one end of the cylinder, reversing direction and moving to the opposite end and reversing back to the starting position is called one cycle.
The leakage will vary from one pump manufacturer to another. Therefore, the system must be calibrated for each particular manufacturers pump. The leakage past the piston occurs constantly, even when no adhesive is being dispensed. Thus, the amount of adhesive used cannot be determined by simply multiplying the swept cylinder volume per cycle by the number of cycles counted.
The system can be used with a variety of pump types, including piston pumps; gear pumps; sliding vane pumps; and centrifugal pumps. However, the system is intended to be used with a pump having significant leakage.
Utilizing a pump with no leakage would drive up the system cost. The pump disclosed above, which is in common use in industry, is designed with a generous clearance between the piston and the cylinder bore. No piston seals are used. The tolerances in manufacturing are large, which keeps the cost low.
Measuring the adhesive flow with a positive displacement flowmeter has been done, but is prohibitively expensive. Moreover, the flowmeter of this type is susceptible to breakdown, and repair requires the filling line to be down for a significant time. In addition, the installation and repair is invasive, in that it requires disassembly of vendor's equipment and installing a third party add-on apparatus.
Accordingly, there is a need for a system capable of monitoring the amount of adhesive delivered to a workpiece by monitoring the strokes of a pump delivering the adhesive.